Coatings are generally applied to optical substrates so as to impart some desired optical characteristic to the optical substrate. For example, an anti-reflection coating might be applied to a surface of an ophthalmic lens so as to reduce parasitic reflections that interfere with the vision of the wearer. Such a coating serves to increase the transmission of light through to the wearer and improve the visibility of the wearer's eyes to others. Another example of a coating used to impart a desired optical characteristic to an optical substrate is that of mirror (reflective) coatings. Such coatings might be applied to a surface of an ophthalmic lens so as to improve the contrast of viewed images for the wearer, as well as to provide a reduction in transmitted ultra-violet or infra red light. Mirror coatings may also impart a coloured appearance to the optical substrate, which may be desirable for aesthetic purposes.
Coatings may also be used to provide an improved physical property to a surface of an optical substrate. For example, abrasion resistant coatings might be applied to a surface of an ophthalmic lens so as to improve the resistance of the surface to mechanical damage, such as scratching.
Scratching of a surface of an ophthalmic lens not only interferes with the vision of the wearer, but it may also result in an article that is less appealing aesthetically. This is particularly the case for lenses that have a mirror coating applied to them. Mirror coatings generally have poor abrasion resistance and as such are easily damaged.
A particular deficiency of mirror coatings is that when scratched, the scratches are highly visible due to the colour contrast between the scratches and the mirror coating. The most dramatic example of such a loss in visual appeal is observed with blue mirror coatings. When scratched, such mirror coatings exhibit highly visible purple, pink and yellow scratches. The visibility of the scratches is due to the difference in colour and brightness between the light reflected from the undamaged surface of the mirror coating and the light reflected from the scratch on the surface of the mirror coating.
Mirror coatings are most often applied to the surface of abrasion resistant coatings such as hardcoat layers. However, although the abrasion resistant coating may provide resistance to mechanical damage to the optical substrate, the mirror coating itself may still be easily damaged. Consequently, any damage to the optical substrate will first occur to the surface of the mirror coating, resulting in a reduction in the quality of the vision of the wearer and an optical substrate that has less aesthetic appeal.
A deficiency in the prior art has been the inability to formulate a coating that provides a reduction in the visibility of scratches on the surface of an optical substrate, and in particular, the inability to formulate mirror coatings that provide a reduction in the visibility of scratches on the surface of an optical substrate. It is also desirable to provide a coating with improved resistance to scratching, as compared to the resistance to scratching of other similar coatings that when damaged show highly visible scratches.